1. Field of the Invention
The present invention relates to a radio communication system and a radio communication method for performing radio communication by combining the OFDM modulation method and the CDMA modulation method, and to a transmitter apparatus and a receiver apparatus suitable for use in the system and the method
2. Description of the Related Art
The “orthogonal frequency and code division multiplexing” (OFCDM) radio communication method has been conventionally known as a radio communication method which combines the “orthogonal frequency division multiplexing” (OFDM) modulation method and the “code division multiple access” (CDMA) modulation method.
The OFCDM radio communication method is based on the multi-carrier CDMA radio communication method, which is now under study for application to a digital mobile communication system.
The multi-carrier CDMA radio communication method is a method of duplicating information symbols along the direction of a frequency axis, multiplying each of the duplicated information symbols by one chip of a spreading code, and then transmitting the chips after spreading in parallel on a plurality of sub-carriers having different frequencies.
In other words, according to the multi-carrier CDMA radio communication method, multiplication using the spreading codes is resultantly executed in the direction of the frequency axis. Therefore, it is possible to realize code multiplexing of a plurality of information channels by use of orthogonal spreading codes.
In addition, according to the multi-carrier CDMA radio communication method, a symbol rate is decreased while a symbol length is increased by parallel transmission using the plurality of sub-carriers. Accordingly, it is possible to reduce an influence of “multi-path interference” which poses a problem in a radio communication environment.
Here, “multi-path interference” is deterioration in transmission quality of radio communication signals, which is attributable to mutual interference of radio communication signals transmitted from a transmitter apparatus because the radio communication signals pass through a plurality of different propagation paths (multiple propagation paths) and thereby reach a receiver apparatus at different timings.
Moreover, the above-described multiple propagation paths cause frequency-selective phasing where fluctuations in conditions in the propagation paths vary depending on the frequency. However, according to the multi-carrier CDMA radio communication method, the radio communication signals are spread along the frequency direction, so that it is possible to improve the transmission quality of the radio communication signals by a frequency diversity effect.
FIG. 1 shows functional blocks in a transmitter apparatus 100 applying the OFCDM radio communication method according to the prior art.
As shown in FIG. 1, the transmitter apparatus 100 applying the OFCDM radio communication method according to the prior art is configured with an information channel signal processing unit 110, an information symbol generating unit 111, an error correction encoding unit 112, a data modulating unit 113, a serial-parallel converting unit 114, symbol duplicating units 115, a spreading signal generating unit 116, multiplying units 117, a signal synthesizing unit 118, a frequency-time converting unit (IFFT) 119, a guard interval inserting unit 120, and an OFCDM outgoing signal outputting unit 121.
The functions of the respective units will be described later. Here, operations of the transmitter apparatus 100 applying the OFCDM radio communication method according to the prior art will be briefly described with reference to FIG. 2.
As shown in FIG. 2, in Step A, the symbol duplicating units 115 of the transmitter apparatus 100 duplicate information symbols generated by the information symbol generating unit 111 along the direction of the frequency axis and the direction of the time axis.
In Step B, each multiplying unit 117 of the transmitter apparatus 100 multiplies each of the information symbols duplicated along the direction of the frequency axis and the direction of the time axis by one chip of a spreading code generated by the spreading signal generating unit 116.
In Step C, the OFCDM outgoing signal outputting unit 121 of the transmitter apparatus 100 performs parallel transmission of the chips after spreading by use of sub-carriers having different frequencies and OFCDM symbols having different timings.
FIG. 3 shows functional blocks of a receiver apparatus 200 applying the OFCDM radio communication method according to the prior art.
As shown in FIG. 3, the receiver apparatus 200 applying the OFCDM radio communication method according to the prior art is configured with an OFCDM incoming signal inputting unit 211, a symbol timing synchronizing unit 212, a guard interval removing unit 213, a time-frequency converting unit (FFT) 214, a spreading signal generating unit 215, multiplying units 216, symbol synthesizing units 217, a serial-parallel converting unit 218, a data demodulating unit 219, an error correction decoding unit 220, an information symbol restoring unit 221, and an outputting unit 223.
The functions of the respective units will be described later. Here, operations of the receiver apparatus 200 applying the OFCDM radio communication method according to the prior art will be briefly described with reference to FIG. 3, FIG. 4A and FIG. 4B.
As shown in FIGS. 4A and 4B, the multiplying units 216 of the receiver apparatus 200 multiply an OFCDM incoming signal received through the OFCDM incoming signal inputting unit 211 by spreading codes generated by the spreading signal generating unit 215, i.e. the spreading codes identical to the spreading codes multiplied in the transmitter apparatus 100, along the direction of the frequency axis (or along the direction of the time axis, or along both of the direction of the frequency axis and the direction of the time axis).
Next, the symbol synthesizing units 217 of the receiver apparatus 200 performs reverse spreading of the OFCDM incoming signal over the respective sub-carriers by synthesizing the signal based on a spreading code cycle.
FIGS. 4A and 4B show an aspect of reverse spreading in which the spreading codes are multiplied only along the direction of the frequency axis.
As shown in FIG. 4A, when fluctuations of propagation paths are constant among the respective sub-carriers, the spreading codes multiplied on the respective information channels are orthogonal to one another. Accordingly, it is possible to restore the signals on the respective information channels completely after reverse spreading.
On the contrary, as shown in FIG. 4B, when the fluctuations of propagation paths vary among the respective sub-carriers, i.e. when the respective sub-carriers are affected by fluctuations of different amplitudes and phases, the signal received after the propagation through the multiple propagation paths loses orthogonalities among the spreading codes. Accordingly, a signal attributable to another information channel remains thereon as a result of interference after reverse spreading and thereby causes a deterioration in transmission quality.
Moreover, particularly when received power is decreased over a plurality of consecutive sub-carriers due to the influence of the multiple propagation paths described above, error correction capability is degraded by the continuing existence of unreliable incoming symbols and thereby causes a deterioration in transmission quality.
A method for solving the problem, i.e. a method for avoiding the continuing flow of unreliable incoming symbols, is known as “interleave”, which involves arranging information symbols with a transmitter apparatus in accordance with a predetermined pattern and rearranging of the information symbols with a receiver apparatus in accordance with a reverse pattern to the predetermined pattern.
However, when “interleave” is applied to the conventional OFCDM radio communication method, the amplitude fluctuations of the sub-carriers within the spreading code cycle are increased and the orthogonalities among the spreading codes are lost, whereby interference from a different information channel is increased.
In such a case, it is important to combine spreading processing along the direction of a frequency axis specific to the OFCDM radio communication method appropriately with “interleave”.
However, application of “interleave” has been so far examined mainly in terms of application to a conventional multi-carrier radio communication method, therefore, application of the “interleave” to the OFCDM radio communication method, which requires consideration for influences of the orthogonalities among the spreading codes, has not been discussed explicitly.
Moreover, the prior Japanese Patent Application No. P2002-190788 only discloses the concept of changing the arrangement of the respective chips after spreading by shifting the chips stepwise in the direction of increasing or decreasing in the carrier frequency along the frequency axis, and has difficulty in estimating the conditions in the propagation paths and in performing rearrangement at appropriate timing based on a result of the estimation.